Project 2 of the ADCS U01 renewal application will be a pivotal multi-site randomized controlled trial to assess whether supervised aerobic exercise is an effective therapeutic option to ameliorate cognitive decline, slow brain atrophy, and favorably alter Alzheimer pathology in older adults with mild cognitive impairment (MCI) and thus in the earliest stages of disease. Two short-term intervention studies completed . thus far provide preliminary support for a cognition-enhancing effect of exercise in MCI. The proposed trial will examine the effects of a structured and supervised 12-month moderate- to high-intensity aerobic exercise intervention versus stretching on cognition and biomarkers of AD pathology in cerebrospinal fluid (CSF) and brain structure using MRI. Sedentary older adults (60-85 yrs old) with amnestic MCI (n=300, CDR=0.5) will be enrolled. Both arms of the intervention will be carried out in community exercise facilities, and physical activity will be gradually increased over the first few weeks and then maintained at the targeted duration and heart rate intensity for the rest of the 12-month trial. Participants will benefit from close supervision using established methods from other successful multi-site exercise trials, on-going health education, and a diversified support system provided by staff, fitness trainers, and an exercise 'Buddy' for peer support. Each week, the interventionist will meet with the participant at the exercise facility to supervise the activity routine, review study goals and progress, identify and resolve issues related to continued adherence, provide encouragement and emotional support, and develop the exercise plan for the coming week. Cognitive and functional outcomes, collected at baseline, months 6 and 12, and again following a 6-month unsupervised extension of the intervention, will include the ADAS-Cog13, supplemented with other tests of executive function, a global (composite) measure of executive function and of episodic memory, the Clinical Dementia Rating scale (sum of boxes). Activities of Independent Living for MCI, and the Geriatric Depression Scale. In addition, participants will complete a brief cognitive test and questionnaire about mood and sleep each week during the supervised exercise session. Biomarkers of exercise efficacy, collected at baseline and month 12, will include measures of aging and AD pathology in CSF, and whole and regional brain volumes. Although regular exercise is widely accepted and recommended to maintain optimum physical health and prevent various medical conditions such as cardiovascular disease and diabetes, exercise is not prescribed with the same conviction to maintain optimum cognitive health or prevent diseases associated with cognitive decline. The results of this study will provide critical data regarding the efficacy of aerobic exercise to improve cognition and favorably alter markers of Alzheimer's disease pathology.